C#中HTML和UBB互相转换的代码
html转UBB的还不是很完美，有空修改，一些代码来自百度谷歌

private string DoHtmlToUBB(string _Html)
        {
 
            _Html = Regex.Replace(_Html, "<br[^>]*>", "\n");
            _Html = Regex.Replace(_Html, @"<p[^>\/]*\/>", "\n");
            _Html = Regex.Replace(_Html, "\\son[\\w]{3,16}\\s?=\\s*([\'\"]).+?\\1", "");
            _Html = Regex.Replace(_Html, "<hr[^>]*>", "[hr]");
 
            _Html = Regex.Replace(_Html, "<(\\/)?blockquote([^>]*)>", "[$1blockquote]");
            _Html = Regex.Replace(_Html, "<img[^>]*smile=\"(\\d+)\"[^>]*>", "'[s:$1]");
            _Html = Regex.Replace(_Html, "<img[^>]*src=[\'\"\\s]*([^\\s\'\"]+)[^>]*>", "");
            _Html = Regex.Replace(_Html, "<a[^>]*href=[\'\"\\s]*([^\\s\'\"]*)[^>]*>(.+?)<\\/a>", "[url=$1]$2[/url]");
            _Html = Regex.Replace(_Html, "<b>(.+?)</b>",@"\[b\]$1\[/b\]");
            _Html = Regex.Replace(_Html, "<[^>]*?>", "");
            _Html = Regex.Replace(_Html, "&amp;", "&");
            _Html = Regex.Replace(_Html, "&nbsp;", " ");
            _Html = Regex.Replace(_Html, "&lt;", "<");
            _Html = Regex.Replace(_Html, "&gt;", ">");
 
            return _Html;
        }
        public string ubbtohtml(string content)  //ubb转html
        {
            content = Regex.Replace(content, @"\r\n", "<br/>");
            content = Regex.Replace(content, " ", "&nbsp;");
            content = Regex.Replace(content, @"\[b\](.+?)\[/b\]", "<b>$1</b>");
            content = Regex.Replace(content, @"\[i\](.+?)\[/i\]", "<i>$1</i>");
            content = Regex.Replace(content, @"\[u\](.+?)\[/u\]", "<u>$1</u>");
            content = Regex.Replace(content, @"\[p\](.+?)\[/p\]", "<p class='load'>$1</p>");
            content = Regex.Replace(content, @"\[align=left\](.+?)\[/align\]", "<align='left'>$1</align>");
            content = Regex.Replace(content, @"\[align=center\](.+?)\[/align\]", "<align='center'>$1</align>");
            content = Regex.Replace(content, @"\[align=right\](.+?)\[/align\]", "<align='right'>$1</align>");
            content = Regex.Replace(content, @"\[url=(?<url>.+?)]\[/url]", "<a href='${url}' target=_blank>${url}</a>");
            content = Regex.Replace(content, @"\[url=(?<url>.+?)](?<name>.+?)\[/url]", "<a href='${url}' target=_blank>${name}</a>");
            content = Regex.Replace(content, @"\[quote](?<text>.+?)\[/quote]", "<div class="quote">${text}</div>");
            content = Regex.Replace(content, @"\[img](?<img>.+?)\[/img]", "<img src='${img}' alt=''/>");
            return content;
        }